Dale R. Walters

POLYAMINES AND PLANT DISEASE

The diamine putrescine and the polyamines spermidine and spermine are found in a wide range of organisms from bacteria to plants and animals. They are basic, small molecules implicated in the promotion of plant growth and development by activating the synthesis of nucleic acids. Polyamine metabolism has long been known to be altered in plants responding to abiotic environmental stress and to undergo profound changes in plants interacting with fungal and viral pathogens. Polyamines conjugated to phenolic compounds, hydroxycinnamic acid amides (HCAAs), have been shown to accumulate in incompatible interactions between plants and a variety of pathogens, while changes in the diamine catabolic enzyme diamine oxidase suggest a role for this enzyme in the production of hydrogen peroxide during plant defence responses. More recent work has suggested a role for the free polyamine spermine in the hypersensitive response of barley to powdery mildew and particularly in tobacco to TMV. The prospects for the genetic manipulation of HCAA levels in plants as a means of both defining their role in plant defence and in the generation of disease resistant plants is discussed briefly.

Controlling crop diseases using induced resistance: challenges for the future

A number of different types of induced resistance have been defined based on differences in signalling pathways and spectra of effectiveness, including systemic acquired resistance and induced systemic resistance. Such resistance can be induced in plants by application of a variety of biotic and abiotic agents. The resulting resistance tends to be broad-spectrum and can be long-lasting, but is rarely complete, with most inducing agents reducing disease by between 20 and 85%. Since induced resistance is a host response, its expression under field conditions is likely to be influenced by a number of factors, including the environment, genotype, crop nutrition and the extent to which plants are already induced. Although research in this area has increased over the last few years, our understanding of the impact of these influences on the expression of induced resistance is still poor. There have also been a number of studies in recent years aimed at understanding of how best to use induced resistance in practical crop protection. However, such studies are relatively rare and further research geared towards incorporating induced resistance into disease management programmes, if appropriate, is required.

Pathogenesis, parasitism and mutualism in the trophic space of microbe-plant interactions

Microbe-host interactions can be categorised as pathogenic, parasitic or mutualistic, but in practice few examples exactly fit these descriptions. New molecular methods are providing insights into the dynamics of microbe-host interactions, with most microbes changing their relationship with their host at different life-cycle stages or in response to changing environmental conditions. Microbes can transition between the trophic states of pathogenesis and symbiosis and/or between mutualism and parasitism. In plant-based systems, an understanding of the true ecological niche of organisms and the dynamic state of their trophic interactions with their hosts has important implications for agriculture, including crop rotation, disease control and risk management.

Antifungal Activities of Four Fatty Acids against Plant Pathogenic Fungi

The effect of the fatty acids linolenic acid, linoleic acid, erucic acid and oleic acid on the growth of the plant pathogenic fungi Rhizoctonia solani, Pythium ultimum, Pyrenophora avenae and Crinipellis perniciosa were examined in in vitro studies. Linolenic and linoleic acids exhibited activity against all of the fungi. However, whereas linolenic acid reduced mycelial growth of R. solani and C. perniciosa at 100μM, the concentration had to be increased to 1000 μM before any effect on mycelial growth of P. ultimum and P. avenae was observed. Linoleic acid only reduced mycelial growth of R. solani, P. ultimum and P. avenae at 1000 μM, but led to a significant reduction in growth of C. perniciosa at 100 μM. In contrast, oleic acid had no significant effect on growth of R. solani or P. avenae, but gave significant reductions in mycelial growth of P. ultimum at 100μM and reduced growth of C. perniciosa significantly at 1000 μM. All of the fatty acids reduced biomass production by all of the fungi significantly in liquid culture when added to the media at 100 μM. Erucic acid had no effect on fungal growth at any concentration examined. The antifungal activities exhibited by linolenic, linoleic and oleic acids may be useful in the search for alternative approaches to controlling important plant pathogens, such as those examined in this study.

Are green islands red herrings? Significance of green islands in plant interactions with pathogens and pests

The term green island was first used to describe an area of living, green tissue surrounding a site of infection by an obligately biotrophic fungal pathogen, differentiated from neighbouring yellowing, senescent tissue. However, it has now been used to describe symptoms formed in response to necrotrophic fungal pathogens, virus infection and infestation by certain insects. In leaves infected by obligate biotrophs such as rust and powdery mildew pathogens, green islands are areas where senescence is retarded, photosynthetic activity is maintained and polyamines accumulate. We propose such areas, in which both host and pathogen cells are alive, be termed green bionissia. By contrast, we propose that green areas associated with leaf damage caused by toxins produced by necrotrophic fungal pathogens be termed green necronissia. A range of biotrophic/hemibiotrophic fungi and leaf‐mining insects produce cytokinins and it has been suggested that this cytokinin secretion may be responsible for the green island formation. Indeed, localised cytokinin accumulation may be a common mechanism responsible for green island formation in interactions of plants with biotrophic fungi, viruses and insects. Models have been developed to study if green island formation is pathogen‐mediated or host‐mediated. They suggest that green bionissia on leaves infected by biotrophic fungal pathogens represent zones of host tissue, altered physiologically to allow the pathogen maximum access to nutrients early in the interaction, thus supporting early sporulation and increasing pathogen fitness. They lead to the suggestion that green islands are ‘red herrings’, representing no more than the consequence of the infection process and discrete changes in leaf senescence.

Inhibition of polyamine biosynthesis in fungi

The diamine putrescine, the triamine spermidine and the tetraamine spermine are ubiquitous in nature. In fungi, spermidine appears to be the major polyamine, and changes in concentrations of both putrescine and spermidine are associated with a range of morphological events. Moreover, it is now known that polyamines are essential for cell division and depletion of intracellular polyamines in fungi is lethal. Since putrescine biosynthesis in most fungi appears to occur via the ornithine decarboxylase (ODC) pathway, there has been much interest in the possible fungicidal effects of specific inhibition of fungal ODC. This paper reviews the results of research on the use of ODC inhibitors to control plant and animal pathogens, and examines the effects of such inhibitors on mycorrhizal fungi. More recent work on the use of putrescine analogues as novel fungicidal agents is also reviewed.

Control of foliar diseases in barley: towards an integrated approach

Barley is one of the world’s most important crops providing food and related products for millions of people. Diseases continue to pose a serious threat to barley production, despite the use of fungicides and resistant varieties, highlighting the impact of fungicide resistance and the breakdown of host plant resistance on the efficacy of control measures. This paper reviews progress towards an integrated approach for disease management in barley in which new methods may be combined with existing measures to improve the efficacy of control in the long-term. Advances have been made in genetic mapping of resistance (R) genes and in identifying novel sources of genes in wild barley populations and land races. Marker assisted selection techniques are being used to pyramid R genes to increase the durability of resistance. Elicitors to induce host resistance used in combination with fungicides can provide effective disease control in the field and could delay the evolution of fungicide insensitivity. Traits that may contribute to disease tolerance and escape have been identified and the extent of genetic variation within barley germplasm is being determined. Tools are being developed to integrate the above methods via an assessment of the risk of economic injury occurring from disease to guide decisions on the requirement for fungicide treatment. Barriers exist to the adoption of integrated management approaches from growers and end-users further down the supply chain (e.g. acceptance of variety mixtures) and policy incentives from government may be required for it to be taken up in practice.

Effects of polyamine biosynthesis inhibitors on growth of Pyrenophora teres, gaeumannomyces graminis, Fusarium culmorum and Septoria nodorum in vitro

The effects of a number of polyamine biosynthetic inhibitors on the growth of four species of necrotrophic fungi were examined in vitro . Additions of polyamines were also made to growth media, both alone and in combination with DFMO, the most widely studied inhibitor of polyamine biosynthesis, and their effects on fungal growth were examined. Species-dependent responses to the inhibitors were observed. DFMO was the least effective inhibitor. Some increases in growth were observed after treatment with inhibitors. This is possibly a result of secondary enzyme production or overproduction of the target enzyme, leading to an initial surge of polyamine synthesis and enhanced growth. The effects of DFMO could be reversed by the addition of polyamines to the growth medium.

Ecological Consequences of Plant Defence Signalling

Abstract Plants respond to local attack by pathogens and herbivores with systemic resistance expression. Much has been learned on major signalling cascades underlying local enemy perception, on systemic signals such as plant hormones and small RNAs, and on genes involved in local and systemic resistance expression. Induced resistance offers exciting prospects for using the plants own defences as environmentally friendly means of protecting crops from pests and pathogens. However, many questions still need to be answered to understand the ecology of induced resistance and before artificially induced resistance can serve as a reliable crop protection strategy. Direct activation of defences is costly and possibly causing yield reductions, particularly if enemy pressure is low. Such costs likely make directly induced resistance unpopular. In contrast, priming triggers defences only following pest or pathogen challenge and is, thus, a less wasteful use of resources. Induced resistance to herbivores and pathogens may benefit crops in the field. Much more has to be done, however, to understand how abiotic and biotic factors such as soil nutrients and the presence of enemies and of mutualistic micro-organisms affect the consequences of a certain resistance induction for plant health, growth, and yield.

The effects of polyamine biosynthesis inhibitors on mycelial growth, enzyme activity and polyamine levels in the oat-infecting fungus Pyrenophora avenae.

SUMMARY: The fungus Pyrenophora avenae, an important pathogen of oat crops, was grown on solid and liquid media containing the polyamine biosynthesis inhibitors difluoromethylornithine (DFMO), methylglyoxal bis(guanylhydrazone) (MGBG), ethylmethylglyoxal bis(guanylhydrazone) (EMGBG) +/- polyamines. All of the compounds inhibited mycelial growth of the fungus. MGBG and EMGBG were more effective than DFMO. The addition of putrescine and spermidine almost completely prevented inhibition of mycelial growth by DFMO. However, no such effect was observed for inhibition by MGBG or EMGBG. Neither the inhibitors nor exogenous polyamines had any significant effect on the size of the fungal cells. DFMO and MGBG, alone and in combination, reduced the activity of ornithine decarboxylase. Fungus grown in media containing EMGBG showed reduced activity of S-adenosylmethionine decarboxylase. Putrescine and spermidine concentrations decreased when the fungus was grown in media containing DFMO or DFMO/MGBG combined. MGBG reduced spermidine and spermine concentrations and EMGBG greatly reduced spermidine concentrations. All of the compounds reduced the concentration of cadaverine, which is a significant component of P. avenae. The respiration rate of the fungus decreased when grown in media containing MGBG or DFMO/MGBG combined.